The present disclosure relates to microscale metamaterials useful, for examples, as sensors. More particularly, it relates to microscale metamaterials exhibiting isotropic properties, and methods of manufacturing the same.
Terahertz (THz) spectroscopy has emerged as an attractive avenue for label-free, fast and versatile detection of chemical and biological substances. Recently, development of high power and long propagation length THz sources has promoted its use for free-space spectroscopy, microstrip line, and metallic mesh methods for the analysis of the biomaterials. Sensors based on the free-space spectroscopy measure changes in dielectric constant due to binding of the molecules. However, this method requires large quantities of the sample materials in order to achieve a reliable response. Microstrip line-based sensors overcome the need for large sample quantities. But, the stronger electric field confinement exists only between the substrate and the strip line, decaying severely in the air region above the strip line. This decay limits the sensitivity of the structure since the high field confinement area is inaccessible to the targeted molecules. Another type of sensors, a metallic mesh based structure, benefits from strong localization of the electromagnetic field at the openings of the mesh and operates by sensing changes in the refractive index near the surface of the metal-air interface. However, the frequency response for these structures resembles that of a high pass filter (no narrow peak exists) with a very low signal transduced at low concentrations of the target molecules. Thus, for higher sensitivity detections, it is necessary to leverage structures that induce a strong coupling between the incident electromagnetic wave and the resonators to deliver sharp edges in the transmission response and create a high field confinement area for detection of the targeted material.
Metamaterials (MMs) are artificial materials that can create unique physical and optical properties unseen in natural materials and that renders them suitable for various applications in sensors, optical devices, plasmonic devices, etc. Terahertz metamaterials (THz MMs) are good candidates as sensors for the detection of chemicals and biomaterials, temperature, strain, alignment, and position. Splitring resonator (SRR)-based metamaterial structures have been extensively studied because of the behavior of relatively sharp edges as well as their ability to manipulate electromagnetic waves and strong confinement of the magnetic field (H) within the arms of the resonator and the electric field (E) confinement within the split. The split contributes capacitance to the resonance frequency which is directly proportional to the relative permittivity. The confinement of electric field within the capacitance controlling split make it a hotspot that has a higher sensitivity than the surrounding areas where the electric field is much weaker. Hence, when a SRR is exposed to a biomolecule, a large change in resonance frequency is seen as a function of the relative permittivity of the external molecule near the split.
The dependence of the resonance frequency on the aforementioned parameters has allowed SRRs to be used in a wide range of sensors to detect micro-organisms, strain, dielectric constants, and displacement without the effects of ambient temperature and pressure. Especially, THz SRR-based biosensors offer an attractive avenue for the development of small scale, label-free detectors capable of being introduced orally or intravenously because of their microscale dimensions, which are comparable to that of most micro-organisms, and the non-ionizing effects of the THz radiation.
However, the polarization dependence of a split-ring resonator (SRR) transmission response poses a drawback. When the magnetic field is polarized perpendicular to the split-containing arm of the resonator, the structure is in 1st mode (magnetic resonance). When the electric field is polarized perpendicular to the split-containing arm of the resonator, the structure is in 2nd mode (electric resonance). As the SRR is rotated from 0° to 90°, the 1st mode reduces and the 2nd mode increases, the reverse phenomenon takes places on rotating from 90° to 180°. The transmission at θ=0° (Tθ) decreases as a function of the rotation angle such that T(θ)=1−(1−Tθ)*|cos2 θ|. As a result, an ambiguity in the transmission spectrum is produced, such that variation due to presence of external molecules or rotation of the SRR cannot be discerned. The angle dependent sinusoidal or anisotropic properties of the SRR design limits their application as sensors when the orientation of the resonator is difficult to control.